Multicomp Waveform Generator External Triggering

Hi Noah
I am lost, completely.
You have gone from 4.8V to 1.56 including overshoot on the leading edge. Take that out and you have a pulse of ≈ 1.2V P-P. just what are you measuring.
If you are inserting 200V into that attenuator you have probably destroyed it. I draw your attention to the specs
JFW Model #: 50B-050
Type: Manually variable attenuator
Impedance: 50 Ohm
dB Total: 102 dB
dB Step: 1 dB
Frequency Range: DC-1000 MHz
RF Input Power: 1 Watt
RF Connectors: BNC, SMA, TNC
With particular attention to “RF input power”
Now if that 200V P-P was a sine wave this would equate to ≈ 100W RMS. Now the fact that this pulse is ≈ 150mS at a fair slow rate might have helped here. I have used this type of thing many times and they are designed to attenuate the output of a signal generator or other low power driving sources, usually in the mW range. NOT to accommodate 200V.

What is that black co-ax cable doing. I assume that is the attenuator output. If it is not terminated in 50Ω it would be the cause of that overshoot you see on the scope screen.

I don’t know anything about your 200V source either. It would need to supply 4A or the 50Ω load of the attenuator would kill it. I haven’t seen any reference to the input level with the attenuator connected.

In other words DO NOT use this type of RF attenuator. Your input voltages will be too high and they need to be terminated or you will have spikes and overshoot everywhere.
Try a simple 2 resistor voltage divider as described above. Observe resistor wattage or you will burn the 82k one out.

It would be interesting or essential to see details of your 200V source, might help with other ideas.
Cheers Bob

Hi Noah
Not heard from you in a while. Hope you are still with us on this.

I have put together 3 scenarios for inverting your pulse. Each should work but the first 2 have some cons while I favour the 3rd one for simplicity among other things.

Firstly your input pulse (I assume negative going) needs to be as near 5V P-P as you can get it.
My test pulse from a function generator with the datum at zero volts and negative going pulse.
Amplitude 5V P-P, Pule width 200nS. Pulse rise and fall time 15nS. Pulse repetition frequency 500Hz (about half way between your quoted 10Hz and 1kHz).
Pic below as seen on my oscilloscope.

Next is an Op Amp circuit. Unity gain inverting.

The main Con here is the requirement of a split supply which means you would have to find -5V from somewhere, can be done but a bit inconvenient if this is the only place you will use it.

Single supply solution.
First the addition of a 1µF capacitor in series. The resultant Pic here

Here the cap has removed the DC component and actually produced an AC signal. The yellow trace is the input to capacitor and the blue is output. If you look carefully you will note the upper bit has gone to 117mV positive WRT 0V (Ground)This is expected as the Cap has moved Ground to the AVERAGE voltage of the input.
Now this is not much use as it stands so we connect a 10kΩ resistor from this Cap output to VCC. This “biases” this average voltage to VCC value (5V). This shown in the next pic.

Now we have something useful. The blue trace has jumped to a bit over 5V and the pulse going close to ground.
We can now put this through an inverting amplifier as before but without the negative 5V supply.

Now there is a major disadvantage with the use of OpAmps here and that is frequency response and importantly pulse rise and fall time. Unfortunately most of the devices commonly available are Audio types and there is one CA3130 ($6.95) but has the disadvantage at working best an above 8.5V. A bit inconvenient, although quoted at 15MHz the pulse rise time is still pretty poor rise time 0.09µS and settling time of 1.2µS.

There is however an alternative i would use. An inverting Schmidt Trigger. very simple to use, pulse in and inverted pulse out. No extra components. Everything is done for you on chip. You will get almost 5V to almost ground with a rise and fall time of about 20nS. These devices are designed to clean up slow rise and fall signals and generally clean up.Circuit below.


One down side here they are commonly in Hex packages so you will have 5 left over. Ground (or connect to VCC) all unused INPUTS. Do nothing with unused outputs.
I say commonly available as I am looking at a Jaycar catalog as somewhere this sort of thing is available over the counter. They have 74SL14 and 74HC14 which are slightly different but either should do. There may be single or dual units around but I don’t know of any off hand.
I have not tried this as I don’t have a Schmidt Trigger on hand and my nearest Jaycar is 30kM away so I have not been there for a while.

If your pulse is too narrow you may have to stretch it. This will probably need a 555 which is not hard to do with the cap (1µF to about 10nF} value change, the addition of another 10kΩ resistor and a timing resistor and capacitor and a diode or 2 I think… I think the 555 would be fast enough but there might be some faster models around these days. I have not had reason to look
Cheers Bob

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Hi Robert!
I’m still here, working on a few other things as well and haven’t been able to work on it too much last week.
These look fantastic and I will try them as soon as I can source the parts. I will probably start with the inverting Schmidt trigger as in my experience, the simpler something is the less there is to go wrong. I’ll certainly look at the others should Schmidt not work

I will keep in touch regarding how these circuits go and hopefully, we crack it sometime in the next 2 weeks.

I appreciate all the help you’ve given me, is there any way I can send a gift your way as a thank you? I work in the Newcastle area if you’re anywhere nearby, but we can also easily mail nationally.

Hi Noah
I would personally go with the schmidt trigger. Preferably the 74HC14 as you will get a greater output swing closer to the rail voltage than the 74LS14. And a good rise and fall time. Available over the counter at Jaycar. I think this will be far better than Op Amp with relatively slow rise and fall times and you would need rail to rail Amps anyway. I think the simplicity offsets the redundant 5 inverters and the 14 pin package.

I am Central Coast so will message privately.
Cheers Bob

Hey Robert!
Thanks for your help, as we discussed I’ll sign off the success here!
The Schmidt trigger was a success using the values you listed and the 74HC14. Just a little bit of noise I will be able to sort out by tweaking the power supply, grounds and circuitry but it finally is triggering. Thanks a ton!

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